<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin:0;
            padding:0;
        }
       .box{
         /* 计算 */
         /* width:calc(100% - 20px);
         height:200px; */

         /* height:100%;
         width:100%; */
         
         height:50vh;
         width:50vw;
         background-color: red;
       }
    </style>
</head>
<body>
    <div class="box"></div>
</body>
</html>